The present invention relates generally to magnetic resonance (MR) imaging and, more particularly, to a method and apparatus for enhanced magnetic preparation in MR imaging. The present invention is further related to variable rate suppression with elliptical centric phase ordered acquisition of MR data. Additionally, the present invention is directed to an imaging sequence that supports fat suppressed contrast enhanced abdominal scans with improved delineation of vessels in structures as well as a high degree of fat suppression without compromising temporal resolution in contrast enhanced first pass breast imaging applications. The present invention is also applicable to other magnetization preparation schemes such as IR preparation, T2 separation, magnetization transfer, and the like, as well as signal suppression in other targeted tissues.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, or “longitudinal magnetization”, MZ, may be rotated, or “tipped”, into the x-y plane to produce a net transverse magnetic moment Mt. A signal is emitted by the excited spins after the excitation signal B1 is terminated and this signal may be received and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (Gx, Gy, and Gz) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received MR signals are digitized and processed to reconstruct the image using one of many well-known reconstruction techniques.
Angiography is the imaging of flowing blood in the arteries and the veins of a patient. MR angiography (MRA) is an imaging modality that produces images of flowing blood that may be analyzed in the identification and diagnosis of tissue abnormalities and pathologies. Elliptical centric phase ordered acquisition is a widely used MRA imaging technique that is used for the suppression of venous signal as well as increasing immunity to breathing artifacts. In elliptical centric ordering, k-space is reordered such that data is acquired in the order of increasing k-space radial distance. That is, data for the central region of k-space is acquired before the periphery of k-space is acquired. Since a significant contribution to the power spectrum of the acquired signal comes from the central region of k-space, the center of k-space is a major determinant of image contrast. As a result, for contrast enhanced acquisition, the acquisition of the central region at the time of contrast bolus arrival after injection is critical for delineation of the arteries without contamination from venous signal in angiographic applications.
Magnetization preparation is commonly employed in MRI to obtain variable contrast weighting like inversion-recovery preparation, fat suppression, and the like. Typically, a preparation sequence is played out every TR or once every N TRs. One proposed MR angiographic imaging technique with elliptical centric acquisition utilizes uniform, periodic application of magnetic preparation pulses. That is, magnetic preparation pulses are played out once every 15 TRs, 30 TRs, 60 TRs, 120 TRs, 240 TRs, or 480 TRs. It has been suggested that playing out magnetic preparation or fat saturation pulses every 480 TRs or twice per acquisition is optimal. However, images reconstructed from acquisitions with bi-application of fat saturation pulses have shown to have considerable residual fat signal as well as observable ghosting artifacts. Further, it has been shown that playing out fat saturation pulses at a uniform rate even as often as every 20 TRs results in non-uniform fat suppression and ghosting. This non-uniformity in fat suppression and ghosting is particularly problematic for those applications that require a high degree of uniform fat suppression such as breast imaging where it is important for detection of small tumor masses.
Moreover, the acquisition of the center of k-space is also important in a number of applications where a magnetization preparation pulse is required for effectiveness. For example, centric ordering of k-space is important when there is a need for fat suppression with a constraint of minimizing acquisition time. In this case, intermittent fat suppression or magnetization preparation pulses may be used rather than a conventional approach of one magnetization preparation pulse per RF excitation pulse. Some MR applications require fat saturation for improved visualization of vessels and structures. For example, in abdominal imaging, an elliptical centric acquisition enables depiction of the arteries without venous contamination during the first pass of a contrast media injection as well as minimizes motion artifacts. Visualization of small vessels may also be improved by suppressing peritoneal fat signal and signal from fat surrounding the kidneys.
Similarly, in breast imaging applications, suppressing signal from fat considerably improves visualization of small tumors. It is generally well known that upon injection of a contrast media, tumors enhance. Depending on the contrast media washout characteristics, the tumor is either classified as benign or malignant. To improve sensitivity (detection of small tumors) and specificity (ability to classify tissue as normal and lesions as benign or malignant), a high spatial resolution time-resolved sequence with good fat suppression is typically required. A conventional method of effecting fat suppression is to apply a spectrally selective saturation pulse, which is repeated every TR. A drawback of this conventional method is that the scan time is prohibitively increased which negatively affects patient throughput, increases scan time and patient discomfort, and increases the likelihood of patient motion-induced artifacts. Alternatively, k-space acquisitions may be segmented such that fat saturation pulses are applied every N TRs instead of every TR. However, with this technique, it has been shown that steady state effects and differential weighting of the center of k-space due to T1 recovery of the fat signal negatively affects image quality. Further, the effectiveness of fat suppression is also compromised.
It would therefore be desirable to have a system and method capable of tissue suppression with elliptical centric phase ordered acquisition of MR data without negatively affecting patient throughput as well as improving image quality.